His locomotive designs were derived from Churchwood's and included:

• One of his first tasks was to design a new express passenger 4-6-0, keeping within an axle load of 20 tons over the same 14ft 9in driving wheelbase as the Stars. Later versions of the Stars had gone up to 19.4 tons axle load. Working on the 20 ton limit he produced the Castle class four-cylinder 4-6-0 express locomotive, of which 171 were built or converted between 1923-50, being an enlargement of the Star class with some constructional modifications.
  
• 410 Hall and 79 Grange class 4-6-0 mixed traffic locos, which were derived from the Saint 4-6-0 express locomotives.
  
• By 1926 only four bridges on the Paddington-Plymouth route were unable to take an axle load of 22.5 tons. These were upgraded in the summer of 1927 and Collett went to work on on a larger and still more powerful express engine. This produced the King class express locos of 1927-30, and were the heaviest and most powerful 4-6-0's in Britain, made possible by a relaxation in permissable axle loading.

He carried out a major program of workshop re-equipment and greatly improved locomotive construction methods. Greater engineering precision gave a large increase in mileages between overhauls. He extended the use of the GWR Automatic Train Control System to almost all of the important routes and was a member of the 1927 Pringle Committee studying the use of such systems in Britain. From 1926, he modernised the Swindon locomotive testing plant to absorb the maximum power of larger locomotives. His later locomotive policy was rather conservative. However, he was amongst the first to consider complete deiselisation.

Last revised: 28 October 1998